Polyethylene composition for pipe applications with improved sagging properties

Abstract

The present invention relates to a polyethylene composition comprising, a base resin (A) comprisinga copolymer of ethylene and at least one comonomer selected from alpha-olefins having from three to twelve carbon atoms,wherein the ethylene copolymer comprises a low molecular weight component (A-1) and a high molecular weight component (A-2) with the low molecular weight component (A-1) having a lower weight average molecular weight than the high molecular weight component (A-2),(B) carbon black in an amount of 1.0 to 10 wt % based on the total amount of the polyethylene composition, and(C) optional further additives other than carbon black.

Description

BACKGROUND OF THE INVENTION[0001]Pipes constructed from polymer materials have a multitude of uses, such as fluid transport, i.e. the transport of liquids, slurries and gases, e.g. water or natural gas. During transport, it is normal for the fluid to be pressurized. Moreover, the transported fluid may have varying temperatures, usually within the range from about 0° C. to about 50° C. Such pressurized pipes are preferably constructed from polyolefin plastics, usually unimodal or bimodal ethylene plastics such as medium density polyethylene (MDPE; density: 930-942 kg / m3) and high density polyethylene (HDPE; density: 942-965 kg / m3).[0002]The expression “pressure pipe” used herein refers to a pipe which, when used, is subjected to a positive pressure, that is the pressure inside the pipe being higher than the pressure outside the pipe.[0003]Polymeric pipes are generally manufactured by extrusion, or, to a small extent, by injection moulding. A conventional plant for extrusion of polyme...

AI Technical Summary

Benefits of technology

The invention is a polyethylene composition that has low sagging and improved mechanical properties, such as tensile properties and resistance to internal pressure. This is achieved through a specific combination of properties, including viscosity at low shear stress, density, MFR, and shear thinning index. The composition also has a high viscosity eta747, which indicates the presence of high molecular weight polymer chains. The invention can be produced using a pre-polymerization step, which can improve the performance of the catalyst and modify the properties of the final polymer. The polymer melt is extruded through an annular die, which can be arranged in either an end-fed or side-fed configuration.

Problems solved by technology

A problem when manufacturing large diameter pipes, particularly from multimodal polymer material, is that it is difficult to maintain uniform dimensions all over the pipe.

The sagging problem is particularly pronounced for thick-walled large diameter pipes, such as pipes with a wall thickness of at least 60 mm.